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Because of their high altitude, in a very dry part of the atmosphere, noctilucent clouds are rather an enigma. Pekka Parvianien Photo

The thing you do know about clouds is that they're a shade of gray, anywhere from white to black. Something you may also know is that most clouds are 1 to 5 miles high. I'm meteorologist Dave Thurlow from The Mount Washington Observatory.

Maybe you didn't know that some clouds come in a variety of colors and reside on the edges of space. The ones we see most often are between 1 and 5 miles high, the ones that make rain and snow only go up to about a mile or so. But way up there in the stratosphere -- sounds so ominous doesn't it -- are the two highest kinds of clouds under the tug of planet earth's gravity. If you have seen these high clouds before, then you have probably seen polar bears, reindeer or penguins too, because they occur almost exclusively over the poles.

	Nacreus Cloud

The lowest of the highest is the nacreus cloud, with the wonderful common name "Mother of Pearl"...that's what they look like. Clouds that look like this are made of water droplets, which is strange, because they are found at 15 miles high, where the temperature is about 100 below zero. So, somehow unfrozen water exists in nacreus clouds, and there's no telling for sure where it's coming from.

Now, way up at 50 miles are the nighttime luminous clouds, or noctilucent. The source of these is even more mystifying. But the sight of one with its radiant collection of colors and nighttime viewing, makes it almost worth the wintertime trip to the arctic or Antarctic, just to see the highest cloud above earth.

The Weather Notebook is produced by the Mount Washington Observatory in cooperation with New Hampshire Public Radio...funded by The National Science Foundation, and underwritten by Subaru, the beauty of All-Wheel Drive.